The problem with getting books before joining a dojo is that what is written in any particular book may not apply to the style taught by the dojo you eventually join. You could end up with a lot of contradictory information.
If you're trying to teach yourself the physical skills of swordsmanship from a book it's even worse; you could develope some bad habits that may be hard to break. Books are best used to learn about swordsmanship, not to learn swordsmanship itself, or to serve as a memory aid for out-of-class practice once you're under the guidance of a qualified teacher.
That said, if you can keep an open mind and remember that there are multiple ways of doing things, more than one name for things, and often different things being meant by the same word, then there are a few books that can serve as starting points for your study.
I own two of the three books I know of by Obata Sensei, and while the older ones may be out of print you may still be able to find them on Amazon or eBay.
These are: Crimson Steel, Naked Blade, and -- the newest -- Shinkendo.
HTH.
Yours in Budo,
---Brian---